When I got my DE1, I was coming from 15 years with the same E61 HX and 10 years with the same grinder. I was lost. 196 F on my EricS group-head thermometer I knew was close to right for the coffees I buy. The "default" profile was 190 F, which wasn't anywhere near the 200-ish that roaster often quote. I started with 18 g in an 18 g basket. Picked a profile that wasn't too fancy. Stayed at "15+" on my grinder, what I had just pulled shots with. It wasn't good. Bumped the grinder up as I would have on the HX. It still wasn't good. Repeat a few times. Ok, try a different profile. Readjust the grind a few times. Try a different profile. Repeat for a pound of coffee and a sink full of shots.
In retrospect, I was changing too many things at once, too fast.
Finally I decided to pick one profile and stick with it. It probably took me 10 shots to learn how my grinder and machine were interacting. Another three or four to explore temperature and accept that while my old number was 196 F but my new number was 88-90 C. Now that I know how my grinder and machine behave for that general kind of profile, I can dial in grind and dose in 3-5 shots. That's a result of knowing my baseline and getting better at guessing if I'm off by a bump, or a mark or two, based on the flow and taste. Those changes have roughly the same impact for the "normal" profiles, no, short, or long PI.
I've got three bags of unknown, unlabeled coffee coming next week. I'll start at 16 g at "15" on my grinder for medium-light roasts, "14" for light, and "16" for medium, into an 18-g basket. I'll use a profile I like that has slow PI that's generally a few seconds past first drops. I'll see how they pull, shooting for 32-35 g in the cup, however long it takes, then taste and adjust grind, dose, and/or yield.
If I think I've got the grind and dose close but the PI doesn't get any drops, I'll probably lengthen it. If it's running out the bottom of the basket too soon, it's more likely that I prepped poorly or grind/dose is way off. If I switch to a 14 basket and down dose, then I'd shorten the PI inflow rate, but keep the time the same. Less basket volume and less grinds means I need less water for the same level of saturation.
If I've got any left and in feeling adventurous, I might try a very different profile. I know that, for example, a long PI and bloom will need a much finer grind than my current profile to keep pressure up / flow down during the "extraction" phase. I'd go two marks finer, as a guess, and repeat the dial-in, as I'm really making a shot a very different way.
Edit: In answer to your recent question, it was more that lighter roasts are often harder to extract well. This often leads to finer grind, higher temperate, more water, more contact time, or a combination. Even with "sane" profiles (blooming and allonge aren't in that category for me yet), the same principles apply. Cooler, coarser, shorter times tend to extract less. Early, low-pressure infusion and its length are levers you can pull once you're close with a flow-management device.